But the idea of shifting the Commons chamber to Whitehall is opposed by a group of largely Tory traditionalists with support from some ministers who believe the Victorian building should not be left unoccupied, while the SNP argues that parliament should move out of London and embrace electronic voting.

Seven fires broke out in the first 11 months of last year at the building, which has a postwar steam heating system with hot pipes lying next to electrical cables with minimal insulation. Two hundred toilets failed earlier this month and a rabbit was even spotted in one of the car parks.

In the corridors behind the Commons chamber. Photograph: David Levene for the Guardian

Meg Hillier, the Labour chair of the public accounts committee, believes a comprehensive refurbishment is necessary, meaning both houses of parliament would move to temporary accommodation for six years from 2025 at a cost of at least £3.5bn.

“It’s not owned by MPs; it’s just the building where we work. It belongs to the people of Britain, it’s the mother of parliaments, it’s a Unesco world heritage site,” said Hillier, who has the support of senior Conservatives on her committee, which monitors public spending.

Sir Edward Leigh, a Conservative MP, is less convinced, arguing that “the list of dangers are very exaggerated ... this is an iconic building and to close it down for 10 years would be a disaster”. He says it would be possible to refurbish the building piece by piece, and when it comes to the Commons and Lords chambers, move them elsewhere on the historic site.

Signs of serious disrepair on the roof of the palace. Photograph: David Levene for the Guardian

The current Palace of Westminster was built in the 1840s according to designs by architect Sir Charles Barry while the gothic interiors were modelled by Augustus Pugin. Heating, sewage and electrical systems run through its cellars, with pipework so dense it is several feet deep, while 24-hour fire patrols are deemed necessary for ongoing safety.

The Commons chamber was bombed in 1941, forcing MPs to sit in the Lords until it was rebuilt on the orders of Sir Winston Churchill. During that period, asbestos was used widely for insulation. Restoration of the external features, such as the Elizabeth Tower that houses Big Ben and the 160-year-old iron roof has already begun but while they can be undertaken as parliament’s business continues, the internal work is far more complex.

Arguments about how to go about renovating the Palace of Westminster have been raging for several years, with a special joint committee of MPs and peers concluding in 2016 that “there is a substantial and growing risk of a single, catastrophic event”. A vote of MPs has been repeatedly delayed, but is finally due to take place on Wednesday with no party whip imposed, meaning the outcome is uncertain.

An amendment by Hillier calls for “a full and timely decant” of both houses: the most likely plan would see the Commons move to Richmond House on Whitehall, and the Lords to the Queen Elizabeth II conference centre on the other side of Parliament Square.

Westminster repair works in doubt as MPs get vote on £4bn cost

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Meanwhile, Leigh’s supporters are backing a motion put down by Andrea Leadsom, the leader of the Commons, that essentially puts off any decision until 2022, while accepting that action should be taken at some point.

Electrics in the basement of the Houses of Parliament. Photograph: David Levene for the Guardian

Hillier’s amendment calls for a board and a delivery authority to be created, to oversee a costed programme of work, meaning that there would be another vote to decide if MPs would actually move out once costs were set. A compromise scheme proposed by the special committee in 2016 is a partial decant where the Commons and the Lords move off site in succession over 11 years, and was costed at £4.4bn.

An SNP amendment suggests parliament could move out of London entirely, saying that there should be a detailed assessment of relocating parliament “away from the Palace of Westminster”, although moving the legislature outside London has not seriously been examined thus far.

A second amendment from the SNP says that any new Commons chamber should have enough seats for all 650 MPs – the current one only has room for 426 – and enable electronic voting.

They look unlikely to succeed. “It would ruin the atmosphere,” said Leigh, adding: “One more reason not to move out”. As for electronic voting, Hillier argued that “while it sounds good” it could “mean that MPs never had to be in parliament” because their votes could be made by proxies.

Any decision to relocate has also to be approved by the House of Lords, which is due to vote next week. Sources say that peers are keen to get on with restoration works and if the Commons were to vote to do nothing the upper house could vote to demand that action be taken immediately, placing it into conflict with the elected chamber.

Options

Full decant. Commons and Lords both move out of the Palace of Westminster to buildings nearby while total renovation works take place.Estimated cost: £3.5bn over six years.

Partial decant. Move the Commons off site first, then the Lords, renovating each in turn.Estimated cost: £4.4bn over 11 years.

Refurbish on site. Commons and Lords chambers would still have to move, probably to Westminster Hall, during a rolling programme of upgrade works.Estimated cost: £5.7bn over 35 years.

This article was amended on 29 January 2018. Options for a “full decant” include moving the Lords to the Queen Elizabeth II conference centre, not Church House as stated in an earlier version.

The long read: As politicians dither over repairs, the risk of fire, flood or a deluge of sewage only increases. But fixing the Palace of Westminster might change British politics for good – which is the last thing many of its residents want

Photographer David Levene toured the obscure passages and dusty basements of the Palace of Westminster as part of our investigation into its desperate state of disrepair – and parliamentarians’ chronic indecision over how to fix it